fix: Ensure parquet schema arg is propagated to IR

crates/polars-io/src/parquet/read/options.rs

@@ -1,11 +1,11 @@
-use arrow::datatypes::ArrowSchemaRef;
+use polars_core::schema::SchemaRef;
 #[cfg(feature = "serde")]
 use serde::{Deserialize, Serialize};
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
 pub struct ParquetOptions {
-    pub schema: Option<ArrowSchemaRef>,
+    pub schema: Option<SchemaRef>,
     pub parallel: ParallelStrategy,
     pub low_memory: bool,
     pub use_statistics: bool,

crates/polars-lazy/src/scan/parquet.rs

@@ -75,7 +75,7 @@ impl LazyFileListReader for LazyParquetReader {
             self.args.low_memory,
             self.args.cloud_options,
             self.args.use_statistics,
-            self.args.schema.as_deref(),
+            self.args.schema,
             self.args.hive_options,
             self.args.glob,
             self.args.include_file_paths,

crates/polars-mem-engine/src/executors/scan/parquet.rs

@@ -62,11 +62,7 @@ impl ParquetExec {
         // Modified if we have a negative slice
         let mut first_source = 0;
 
-        let first_schema = self
-            .options
-            .schema
-            .clone()
-            .unwrap_or_else(|| self.file_info.reader_schema.clone().unwrap().unwrap_left());
+        let first_schema = self.file_info.reader_schema.clone().unwrap().unwrap_left();
 
         let projected_arrow_schema = {
             if let Some(with_columns) = self.file_options.with_columns.as_deref() {
@@ -262,11 +258,7 @@ impl ParquetExec {
             eprintln!("POLARS PREFETCH_SIZE: {}", batch_size)
         }
 
-        let first_schema = self
-            .options
-            .schema
-            .clone()
-            .unwrap_or_else(|| self.file_info.reader_schema.clone().unwrap().unwrap_left());
+        let first_schema = self.file_info.reader_schema.clone().unwrap().unwrap_left();
 
         let projected_arrow_schema = {
             if let Some(with_columns) = self.file_options.with_columns.as_deref() {

crates/polars-pipe/src/executors/sources/parquet.rs

@@ -261,10 +261,7 @@ impl ParquetSource {
         }
         let run_async = paths.first().map(is_cloud_url).unwrap_or(false) || config::force_async();
 
-        let first_schema = options
-            .schema
-            .clone()
-            .unwrap_or_else(|| file_info.reader_schema.clone().unwrap().unwrap_left());
+        let first_schema = file_info.reader_schema.clone().unwrap().unwrap_left();
 
         let projected_arrow_schema = {
             if let Some(with_columns) = file_options.with_columns.as_deref() {

crates/polars-plan/src/plans/builder_dsl.rs

@@ -85,7 +85,7 @@ impl DslBuilder {
         low_memory: bool,
         cloud_options: Option<CloudOptions>,
         use_statistics: bool,
-        schema: Option<&Schema>,
+        schema: Option<SchemaRef>,
         hive_options: HiveOptions,
         glob: bool,
         include_file_paths: Option<PlSmallStr>,
@@ -109,7 +109,7 @@ impl DslBuilder {
             file_options: options,
             scan_type: FileScan::Parquet {
                 options: ParquetOptions {
-                    schema: schema.map(|x| Arc::new(x.to_arrow(CompatLevel::newest()))),
+                    schema,
                     parallel,
                     low_memory,
                     use_statistics,

crates/polars-plan/src/plans/conversion/dsl_to_ir.rs

@@ -145,18 +145,31 @@ pub fn to_alp_impl(lp: DslPlan, ctxt: &mut DslConversionContext) -> PolarsResult
                 let mut file_info = match &mut scan_type {
                     #[cfg(feature = "parquet")]
                     FileScan::Parquet {
+                        options,
                         cloud_options,
                         metadata,
-                        ..
                     } => {
-                        let (file_info, md) = scans::parquet_file_info(
-                            &sources,
-                            &file_options,
-                            cloud_options.as_ref(),
-                        )
-                        .map_err(|e| e.context(failed_here!(parquet scan)))?;
-                        *metadata = md;
-                        file_info
+                        if let Some(schema) = &options.schema {
+                            // We were passed a schema, we don't have to call `parquet_file_info`,
+                            // but this does mean we don't have `row_estimation` and `first_metadata`.
+                            FileInfo {
+                                schema: schema.clone(),
+                                reader_schema: Some(either::Either::Left(Arc::new(
+                                    schema.to_arrow(CompatLevel::newest()),
+                                ))),
+                                row_estimation: (None, 0),
+                            }
+                        } else {
+                            let (file_info, md) = scans::parquet_file_info(
+                                &sources,
+                                &file_options,
+                                cloud_options.as_ref(),
+                            )
+                            .map_err(|e| e.context(failed_here!(parquet scan)))?;
+
+                            *metadata = md;
+                            file_info
+                        }
                     },
                     #[cfg(feature = "ipc")]
                     FileScan::Ipc {

crates/polars-stream/src/nodes/parquet_source/mod.rs

@@ -156,18 +156,7 @@ impl ComputeNode for ParquetSourceNode {
             eprintln!("[ParquetSource]: {:?}", &self.config);
         }
 
-        self.schema = Some(
-            self.options
-                .schema
-                .take()
-                .unwrap_or_else(|| self.file_info.reader_schema.take().unwrap().unwrap_left()),
-        );
-
-        {
-            // Ensure these are not used anymore
-            self.options.schema.take();
-            self.file_info.reader_schema.take();
-        }
+        self.schema = Some(self.file_info.reader_schema.take().unwrap().unwrap_left());
 
         self.init_projected_arrow_schema();
         self.physical_predicate = self.predicate.clone().map(phys_expr_to_io_expr);

py-polars/tests/unit/io/test_lazy_parquet.py

@@ -694,6 +694,18 @@ def test_parquet_schema_arg(
         pl.DataFrame({"1": None, "a": [1, 2], "b": [1, 2]}, schema=schema),
     )
 
+    # Issue #19081: If a schema arg is passed, ensure its fields are propagated
+    # to the IR, otherwise even if `allow_missing_columns=True`, downstream
+    # `select()`s etc. will fail with ColumnNotFound if the column is not in
+    # the first file.
+    lf = pl.scan_parquet(
+        paths, parallel=parallel, schema=schema, allow_missing_columns=True
+    ).select("1")
+
+    s = lf.collect(streaming=streaming).to_series()
+    assert s.len() == 2
+    assert s.null_count() == 2
+
     # Test files containing extra columns not in `schema`
 
     schema: dict[str, type[pl.DataType]] = {"a": pl.Int64}  # type: ignore[no-redef]